1. Field of the Invention
The present invention relates to devices for monitoring patient temperature in a hospital environment.
2. Description of the Prior Art
U.S. Pat. No. 3,921,621 discloses a patient temperature monitoring system which utilized discrete integrated ciruit and analog components. Devices constructed according to that patent reduced considerably the time necessary for hospital nurses to monitor and record the temperatures of patients in their wards. However, the present invention has several features not found in prior patient temperature monitoring systems.
One object of the present invention is to provide a patient temperature monitoring system which can calculate and selectively display patient temperatures in either a Fahrenheit or Centigrade convention.
Another object of the invention is to provide a patient temperature monitoring system in which only one transmitter at a time is actuated. This is achieved by equipping the receiver, which is a portable unit carried by a nurse, with an antenna having an actuator affixed to the antenna. The actuator is merely brought into proximity of the transmitter associated with the patient whose temperature is to be monitored. The actuator closes a switch within the transmitter to provide power to the transmitter output unit. In the absence such switch closure, the switch is normally open and there is no power drain upon the transmitter batteries. Conventional mercury batteries are thereby able to last for years within each transmitter, and do not require recharging or replacement at approximately monthly intervals as do batteries associated with conventional patient temperature telemetry transmitting units.
A further object of the invention is to provide the receiver with a means for ascertaining when the transmitting battery voltage level has dropped to a point where further reduced voltage output will impair operation of the system. This is achieved in the present invention by a comparison function which compares a stored maximum reference duty cycle length with the duration of the reference duty cycle of the transmitter output measured during a test procedure. As the battery voltage level in the transmitter drops over a period of time, the duration of the reference duty cycle of the transmitter output attributable to the reference resistance will increase, even though the duration of thermistor portion will vary in accordance with patient temperature. When the total duration of the reference duty cycle of the transmitter increases to a maximum limit, the user of the receiver is informed that the transmitter battery level is low.
Another object of the invention is to provide a means for self-calibration of the receiver. The receiver includes a test circuit which is operated by a manual switch. Actuation of this switch to the test mode substitutes an internal reference oscillator output for the transmitter output. The internal reference oscillator is set to a predetermined duty cycle to produce a specific temperature indication in the display, such as 98.6.degree. F. Deviation from this predetermined temperature indication informs the user that the instrument is out of calibration.
Yet a further object of the invention is to provide a transmitter for each patient in which the thermistor and reference resistor are encompassed together within a disposable packet. The packet is a plug-in module and may be easily disconnected from the transmitter case. This avoids the necessity for sanitizing the packet containing the thermal resistor and thermistor elements, and instead allows these expendable components to be discarded and replaced with an identical, modular unit.